Assessing and Measuring Wetland Hydrology - Wetland Techniques - page 128

0

0.2

0.4

0.6

0.8

1

0 0.5 1

normalized velocity

Fig. 3.14 Typical velocity profile plotting dimensionless velocity normalized to the profile-mean
value against dimensionless depth normalized to the total depth of profile. At a normalized depth
of 0.6, normalized velocity is approximately equal to 1 (Adapted from Rantz 1982 :133. Published
with kind permission of the U.S. Geological Survey. Figure is public domain in the USA. All
Rights Reserved)

narrow channel, standing on an elevated plank crossing the channel (Rantz
1982 :146). A cable with a heavy depth-sounding weight at the bottom is used to
measure depth and suspend the current meter at the proper depth when
measurements are made from a boat or bridge or cableway (Rantz 1982 :101-102).
Flow velocity in a channel also varies vertically. Within a rectangular subsec-
tion, the velocity is very small at the streambed and increases upwards. Figure 3.14
shows the standard velocity profile used by the U.S. Geological Survey (Rantz
1982 :133) based on intensive investigation of open channel hydraulics. In this
profile, the velocity measured at a normalized depth of 0.6 (six tenths of the
distance from the water surface to the bed) is almost identical to the profile-mean
velocity, and also the arithmetic mean of two velocity values measured at
normalized depths of 0.2 and 0.8 is almost identical to the profile-mean velocity.
Based on these observations, it is common practice to represent the mean velocity
of a subsection by the arithmetic mean of two measurements at 0.2 and 0.8 depths
(2-point method), or a single velocity measurement using a current meter placed at
0.6 depth (six-tenth method). The 2-point method generally gives more reliable
results than the six-tenth method, but in shallow streams making a measurement at
0.2 depth may not be possible, depending on the size and specification of the current
meter, in which case the six-tenth method is preferred (Rantz 1982 :134-135).
An alternative approach is to use a current meter that integrates velocity
measurements and calculates a profile-mean velocity as the meter is continuously
moved up and down the entire profile at a uniform rate for a sufficiently long period
(integration method).
Several types of current meters are commonly used to make streamflow measure-
ments. Mechanical current meters have long been the sensor of choice, but electro-
magnetic current meters and acoustic Doppler velocimeters (ADV; Winter 1981 )